In the prior art, electrically powered lamps, for example, arc lamps, are commonly used to illuminate a movie scene, a theatrical set, a musical production, or the like. The lamp is typically mounted in an enclosure adjacent a reflecting mirror and may or may not have a to Fresnel lens mounted on the enclosure. Typically, the reflecting mirror and/or bulb is movably mounted so as to permit the light cast by the lamp to either "flood" a scene or merely to cast a "spot" of light or something between these two extremes.
The arc lamp used in such applications is usually rated at several hundred to tens of thousands watts and is typically energized by a power supply or ballast which in turn is connected to a conventional source of electrical power. Most ballasts and power supplies are not controllable so that the color of light produced by the arc lamp cannot be controlled directly such as by changing the power delivered to lamp. When the arc lamp is being used to illuminate a movie scene, for example, and color film is being used to shoot the movie, it is very important that the color of the light emitted by the arc lamps be properly balanced against the particular film type being used. If there is a mis-match, the scene will either appear too green or too red to the viewer of the movie. Since several lamps are generally used to illuminate a scene, it is important that all of the lamps produce the same color of light so that the scene coloring and actor's coloring does not change as the camera moves about the scene. In the prior art, the color of the light emitted by arc lamps has been controlled manually by using appropriate filters (usually of the `gel` type) or, less frequently, by using a manually controllable power supply. The person doing the controlling either relies on his or her eye to properly adjust the color of the arc lamp, or alternatively, uses a hand held color meter, such as the commonly used Minolta meter, to test for the color of light emitted from lamp and filter combinations used to illuminate the scene then and adjusts either the filter or filters or the power suply (if controllable) as necessary to properly color balance the light with the film being used. Typically, when arc lamps are being used to illuminate the scene, the proper color balance occurs at 5600.degree. K. color temperature.
Variations in age and conditions of the lamps result in variations in color temperature output from lamp to lamp. These variations in color make the manual task of color balancing the lamps and filters quite important since a color mismatch is quite detracting to the artistic quality of the scene. Moreover, proper color balance is difficult to keep within acceptable limits with present manual techniques. The present invention is concerned with automating the color balance of such light source.